http://www.ck12.org Chapter 16. Solutions
- mass of CaCl 2 = 82.20 g
- molar mass of CaCl 2 = 110.98 g/mol
- mass of H 2 O = 400. g = 0.400 kg
- Kf(H 2 O) =−1.86 °C/m
- Kb(H 2 O) = 0.512 °C/m
- CaCl 2 dissociates into 3 ions
Unknown
- Tf=? °C
- Tb=? °C
First, calculate the moles of CaCl 2. This result is then used to calculate the molality of the solution. The freezing
and boiling points are determined after, including multiplying by 3 for the three ions.
Step 2: Solve.
82 .20 g CaCl 2 ×
1 mol CaCl 2
110 .98 g CaCl 2
= 0 .7407 mol CaCl 2
0 .7407 mol CaCl 2
0 .400 kg H 2 O
= 1. 85 mCaCl 2
∆Tf= Kf×m×3 = -1.86°C/m×1.85m×3 = -10.3°C
Tf= -10.3°C
∆Tb= Kb×m×3 = 0.512°C/m×1.85m×3 = 2.84°C
Tb=102.84°CStep 3: Think about your result.
Since the normal boiling point of water is 100.00°C, the calculated result for∆Tbmust be added to 100.00 to find
the new boiling point.
Practice Problem- Calculate the freezing and boiling points of a solution prepared by dissolving 20.0 g of Al(NO 3 ) 3 in 100.0 g
of water.
Calculating Molar Mass
In the laboratory, freezing point or boiling point data can be used to determine the molar mass of an unknown
solute. The Kfor Kbof the solvent must be known. It also must be known whether the solute is an electrolyte or a
nonelectrolyte. If the solvent is an electrolyte, you would need to know the number of ions that are produced when
it dissociates.
Sample Problem 16.8: Molar Mass from Freezing Point Depression
38.7 g of a nonelectrolyte is dissolved into 218 g of water. The freezing point of the solution is measured to be
−5.53°C. Calculate the molar mass of the solute.
Step 1: List the known quantities and plan the problem.
Known
- ∆Tf=−5.53°C